efi: Makefile: Export variables for use with EFI

When building an EFI app we need three things:

   - start-up code
   - relocation code
   - link script

These are all different for each architecture. We also need special
compiler flags in some cases.

Add top-level Makefile variables for these along with documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>